1. Field of the Invention
The present invention relates to an improvement of magnetic field generating apparatus for use in a magnetic resonance imaging apparatus (hereinafter referred to as MRI) for medical treatment use and the like and, more in particular, it relates to an MRI magnetic field generating apparatus having an enhanced magnetic efficiency, decreased in the size of the magnetic field generating apparatus and capable of attaining a reduced cost by optimizing the material and the arrangement of permanent magnets as a magnetic field generating source.
2. Description of the Related Art
MRI is an apparatus for obtaining tomographic images of an object by inserting a person to be diagnosed in a magnetic space of a magnetic field generating apparatus that forms a strong magnetic field, and constitution using permanent magnets as magnetic field generating sources as shown in FIG. 7 and FIG. 8 have been known.
In the MRI magnetic field generating apparatus shown in FIG. 7, a pair of permanent magnets 2a, 2b each of a trapezoidal section are opposed in parallel with each other in hexagonal cylindrical yokes 1 with a predetermined gap being defined between each of pole faces of them, and permanent magnets 3a, 3b, 3c, 3d each of a triangular section are disposed in adjacent with both sides of the trapezoidal permanent magnets 2a, 2b, such that a magnetic space 4 having a hexagonal section in the direction perpendicular to the longitudinal direction is defined by the pole faces of the trapezoidal permanent magnets 2a, 2b and the pole faces of the triangular permanent magnets 3a, 3b, 3c, 3d, thereby forming a magnetic field in the direction Y of the drawing in the magnetic field space (refer to Japanese Patent Laid Open Hei 5-287047).
In this constitution, for forming the magnetic field in predetermined direction, the trapezoidal permanent magnets 2a, 2b each of the trapezoidal section have directions of magnetization identical with the direction of magnetic field in the space, while the permanent magnets 3a, 3b, 3c, 3d each of the triangular section have directions of magnetization at a 90.degree. direction (perpendicular direction) relative to the direction of magnetic field in the space, and a rare earth type permanent magnet having a large maximum energy product ((BH)max) such as an Fe--B--R type permanent magnet has been used for each of the permanent magnets with an aim of reducing the size and the weight.
Further, for improving the uniformity of the magnetic field in the magnetic field space 4 in the MRI magnetic field generating apparatus having the foregoing constitution. It has also been proposed a constitution in which pole pieces each made of a predetermined material and having a predetermined shape (not illustrated) are disposed on the counterfaces of a pair of permanent magnets 2a, 2b each of the trapezoidal section to the magnetic space 4 (Japanese Patent Laid Open Hei 5-291026).
In the MRI magnetic field generating apparatus shown in FIG. 8, a pair of plate yokes 11a, 11b are opposed to each other by way of a plurality (usually four) of post yokes 13 so as to define a predetermined magnetic field space 12 and permanent magnetic 14a, 14b are disposed on the counterfaces of the plate yokes 11a, 11b to the magnetic field space 12 to define a magnetic field in the direction Y of the drawing in the magnetic field space 12. In the drawing, pole pieces 17 are disposed with an aim of further improving the uniformity of the magnetic field in the magnetic field space 12.
The illustrated constitution shows magnetic field generating apparatus having an enhanced magnetic efficiency, decreased in the size and capable of attaining reduced cost by coaxially arranging rare earth system permanent magnets 15a, 15b having a large maximum energy product ((BH)max) such as Fe--B--R system permanent magnets and ferrite system permanent magnets 16a, 16b having a smaller maximum energy product ((BH)max) than that of the rare earth system permanent magnets 15a, 15b but of greatly reduced cost (Japanese Utility Model Publications Hei 3-14011 and Hei 3-14012).
In the constitution described above, each of the magnetization for the rare earth permanent magnets 15a, 15b and the ferrite permanent magnet 16a, 16b has the identical direction with that of the magnetic field in the magnetic field space 12.
Descriptions have been made to the constitution of typical magnetic field generating apparatus using permanent magnets as the magnetic field generating source. For attaining the generalized use of the MRI magnetic field generating apparatus, it has been demanded for further decreasing in the size and the weight and reducing the cost, and an improvement has been made recently for the magnetic field generating apparatus of a constitution shown in FIG. 7 with a view point, for example, of less magnetic leakage and higher magnet efficiency as compared with the constitution shown in FIG. 8.
However, for the magnetic field generating apparatus of the constitution shown in FIG. 7, consideration has been given only to the constitution having only the rare earth system permanent magnets of a large maximum energy product ((BH)max) as explained previously and not to an arrangement combined with inexpensive permanent magnets such as ferrite system permanent magnets as in the constitution of the magnetic field generating apparatus shown in FIG. 8.
Since the magnetic field generating apparatus of the constitution shown in FIG. 7 and the magnetic field generating apparatus of the constitution shown in FIG. 8 are different from each other in the basic technical concept regarding the magnetic path, no aimed magnetic field of a high uniformity can be obtained in the magnetic field space, by merely replacing the rare earth system permanent magnets with the ferrite system permanent magnets in one of the permanent magnets 2a, 2b each of the trapezoidal sections and the permanent magnets 3a, 3b, 3c, 3d each of the triangular sections, which only results in increased size of the apparatus.